Brain-derived neurotrophic factor (BDNF) is a small secreted protein that plays a fundamental role in nervous system development and in regulating the strength of existing synapses throughout the adult life. Imbalances in BDNF signaling impair several forms of synaptic plasticity and lead to a wide range of cognitive abnormalities. Unlike the classical neurotransmitters, BDNF is secreted by membrane-trafficking vesicular organelles that undergo exocytosis in neuronal processes and postsynaptic spines. The polymorphism in human BDNF gene which selectively abolishes activity-dependent synaptic release of BDNF has been associated with deficits in learning and memory. Remarkably, despite the importance of BDNF in brain development and plasticity, the molecular mechanisms underlying BDNF secretion have not been elucidated. Syt-11 is a member of synaptotagmin family of secretory proteins that are known to regulate exocytosis of various trafficking organelles. Recent genetic studies linked Syt-11 to familiar schizophrenia. The new observations in this proposal implicate Syt-11 to BDNF secretion. Specifically, we show that: i) Syt-11 is exclusively expressed in neurons and is localized on vesicular organelles that undergo activity-dependent exocytosis; ii) Syt-11 co-localizes with BDNF; iii) mouse Syt-11 gene is essential for survival during postnatal development; and iv) genetic deletion of Syt-11 impairs activity- dependent secretion of BDNF and homeostatic synaptic plasticity. Based on these observations we hypothesize that Syt-11 resides on and regulates exocytosis of trafficking vesicles that transport and release BDNF in neurons. This central hypothesis will be tested by several approaches. By using the subcellular fractionations and high-resolution live cell imaging, we will determine whether Syt-11 and BDNF co-traffic in the same secretory vesicles. Importantly, we will identify the sites of vesicle exocytosis and determine how exocytosis correlates with neural activity. As the next step, we will determine the extent to which transport and secretion of BDNF depends on Syt-11, and on interactions of Syt-11 with its effectors. This goal will be accomplished by analyses of subcellular distribution and secretion of BDNF in Syt-11 deficient neurons. Finally, we will perform electrophysiological analyses of cultured neurons and acute slices to test whether genetic deletion of Syt-11 impairs synaptic transmission and BDNF-dependent synaptic plasticity. These studies will provide new significant insights into cellular and molecular mechanisms underlying neurotrophin signaling in brain. Importantly, these studies will elucidate a secretory pathway that when defective causes abnormalities in synaptic and cognitive functions